The present invention is generally related to an inkjet printer printhead having an improved orifice design and is more particularly related to a printhead orifice design having an opening with characteristics producing reduced ink spray and improved trajectory error.
An inkjet printer forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land in desired locations on the medium. In its simplest form, such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet printer comprises an ink containment section, which stores and supplies ink as needed, and a printhead, which heats and expels the ink droplets as directed by the printer control software. Typically, the printhead is a laminate structure including a semiconductor base, a barrier material structure which is honeycombed with ink flow channels, and an orifice plate which is perforated with small holes or orifices arranged in a pattern which allows ink droplets to be expelled.
In one variety of inkjet printer the expulsion mechanism consists of a plurality of heater resistors formed in the semiconductor substrate which are each associated with one of a plurality of ink firing chambers formed in the barrier layer and one orifice of a plurality of orifi in the orifice plate. Each of the heater resistors is connected to the controlling software of the printer such that each of the resistors may be independently energized to quickly vaporize a portion of ink into a bubble which subsequently expels a droplet of ink from an orifice. Ink flows into the firing chamber formed in the barrier layer around each heater resistor and awaits energization of the heater resistor. Following ejection of the ink droplet and collapse of the ink bubble, ink refills the firing chamber to the point where a meniscus is formed across the orifice. The form and constrictions in barrier layer channels through which ink flows to refill the firing chamber establish both the speed at which ink refills the firing chamber and the dynamics of the ink meniscus. Further details of printer, print cartridge, and printhead construction may be found in the Hewlett-Packard Journal, Vol. 36, No. 5, May 1985, and in the Hewlett-Packard Journal, Vol. 45, No. 1, February 1994.
One of the problems faced by designers of print cartridges is that of maintaining a high print quality while achieving a high rate of printing speed. When a droplet is expelled from an orifice due to the rapid boiling of the ink inside the firing chamber, most of the mass of the ejected ink is concentrated in the droplet which is directed toward the medium. However, a small portion of the expelled ink resides in a tail extending from the droplet to the surface opening of the orifice. The velocity of the ink found in the tail is generally less than the velocity of the ink found in the droplet so that at some time during the trajectory of the droplet, much of the tail is severed from the droplet. Some of the ink in the severed tail rejoins the expelled droplet or remains as a distortion of the droplet to create rough edges on the printed material. Some of the expelled ink in the tail returns to the printhead, forming puddles on the surface of the orifice plate of the printhead. Some of the ink in the severed tail forms subdroplets (xe2x80x9csprayxe2x80x9d) which travel and spread randomly in the general direction of the ink droplet. This spray often lands on the medium to produce a background of ink haze.
To reduce the detrimental results of spray, others have reduced the speed of the printing operation but have suffered a reduction in the number of pages which a printer can print in a given amount of time. The spray problem has also been addressed by optimizing the architecture or geometry of the ink firing chamber and the associated ink feed conduits in the barrier layer. Orifice geometries also affect spray, see U.S. patent application Ser. No. 08/608,923, xe2x80x9cAsymmetric Printhead Orificexe2x80x9d filed on behalf of Weber et al. on Feb. 29, 1996 now still pending.
One conventional method of fabricating an orifice plate utilizes an electroless plating technique on a prefabricated mandrel. Such a mandrel is illustrated in FIG. 1 (which is not drawn to scale), in which a substrate 101 has at least one flat surface constructed of silicon or glass. Disposed on the flat surface of the substrate 101 is a conducting layer 103, generally a film of chromium or stainless steel. A vacuum deposition process, such as the planar magnetron process, may be used to deposit this conductive film 103. Another vacuum deposition process may be used to deposit a dielectric layer 105, which typically is silicon nitride, and is deposed by a vacuum deposition process such as a plasma enhanced chemical vapor deposition process. Dielectric layer 105 is desirably very thin, typically having a thickness of approximately 0.30 xcexcm. Dielectric layer 105 is masked with a photoresist mask, exposed to UV light, and introduced into a plasma etching process which removes most of the dielectric layer except for xe2x80x9cbuttonsxe2x80x9d of dielectric material in preselected positions on the conductive layer 103. Of course, these positions are predetermined to be the location of each orifice of the orifice plate which is to be created atop the mandrel.
This reusable mandrel is placed into an electroforming bath in which the conducting layer 103 is established as a cathode while a base material, typically nickel, is established as the anode. During the electroforming process, nickel metal is transferred from the anode to the cathode and the nickel (shown as layer 107) attaches to the conductive areas of the conductive layer 103. Since the nickel metal plates uniformly from each conductive plate of the mandrel, once the surface of the dielectric button 105 is reached, the nickel overplates the dielectric layer in a uniform and predictable pattern. The parameters of the plating process, including the time of plating, are carefully controlled so that the opening of the nickel layer 107 formed over the dielectric layer button 105 is a predetermined diameter (typically about 45 xcexcm) at the dielectric surface. This diameter is usually one third to one fifth the diameter of the dielectric layer button 105 thereby resulting in the top layer of the nickel 107 having an opening at the inner surface of the orifice plate of diameter d2 which is approximately three to five times the diameter of d1 of the opening which will be the orifice aperture at the external surface of the orifice plate. At the completion of the electroless plating process, the newly formed orifice plate is removed from the mandrel and gold plated for corrosion resistance of the orifice. Additional description of metal orifice plate fabrication may be found in U.S. Pat. Nos. 4,773,971; 5,167,776; 5,443,713; and 5,560,837, each assigned to the assignee of the present invention.
Improperly directed ink drops and satellite droplets and spray undesirably result in a poorer quality of character and image formation on an inkjet printed medium. It is desirable that random trajectory due to drops issuing randomly from sides of the ejecting orifice be reduced and that spray due to drop tail break-off be diminished.
The present invention encompasses a printhead for an inkjet printer which utilizes an ink ejector to expel ink from orifi in an orifice plate. The orifice plate has at least one orifice extending through the orifice plate from a first surface of the orifice plate opposite the ink ejector to a second surface of the orifice plate essentially parallel the first surface. The orifice includes an aperture at the second surface, the aperture comprised of at least two intersecting edges defining a periphery of the aperture. A first edge comprises a first arc segment having a first radius and a first center disposed within the periphery of the aperture. A second edge comprising a second arc segment having a second radius and a second center disposed outside the periphery of the aperture.